sql - Can't UNION on primary key -


so these 2 queries:

select * coursegrade;  select * lettergrades;

enter image description here

when union see lettergradeid has lettergrade:

select *      coursegrade     inner join lettergrades     on coursegrade.lettergradeid=lettergrades.lettergradeid;

i'm getting error saying 

invalid column name 'lettergradeid'.

can tell me going wrong?

you misspelled name cg.lettergradedid

select *  coursegrade cg inner join lettergrades lg on cg.lettergradedid=lg.lettergradeid;

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